Fractionation of air to obtain oxygen of about seventy percent purity

ABSTRACT

Oxygen of approximately 70% purity is obtained by a two-stage rectification process in which air is cooled against the fractionation products, part of the cooled air is liquefied against evaporating oxygen product, thus liquefied air is subcooled, expanded and evaporated against vapor of the first rectification column to provide reflux therefor, and the evaporated air is fed to the second column. Bottom liquid of the first column is also used to produce reflux at the top thereof and then discharged into the second column. A process gas stream is expanded to produce refrigeration.

0 United States Patent 1191 111 3,798,917 Juncker Mar. 26, 1974 [54]FRACTIONATION OF AIR TO OBTAIN 2,822,675 2/1958 Grenier 62/38 OXYGEN 0ABOUT SEVENTY PERCENT 3,563,046 2/1971 Van Bausch... 62/29 PURITY2,873,583 2/1959 Potts 62/38 3,260,056 7/1966 Becker.... 62/38 [75]Inventor: Friedrich Juncker, 3,113,854 12/1963 Bernstein.... 62/38Bergen Enkheim Germany Grunberg.... 2,584,985 2/1952 Cicacese..... 62/291 gn Messer Griesheim GmbH, 2,918,802 12/1959 Grunberg 62/38 Frankfurt,Germany [22] Filed: Apr. 27, 1971 Primary Examiner-Norman Yudkoff Appl.No.1 137,887

Assistant Examiner-Arthur F. Purcell Attorney, Agent, or Firm-Paul W.Garbo [57] ABSTRACT Oxygen of approximately 70% purity is obtained by atwo-stage rectification process in which air is cooled against thefractionation products, part of the cooled air is liquefied againstevaporating oxygen product,

8 Claims, 3 Drawing Figures VIII/11,1}-

MTENTEB MAR 26 I974 -SHEET 10F 3 INVENTOR. FRIEDRICH JUNCKER mmgnmzemm3.798317 SHEET 2 0F 3 AIR INVENTOR. FRIEDRICH JUNCKER BY Wm AGENTmgmgumzs i974 3.798.917

SHEET 3 or 3 AIR INVENTOR. FRIEDRICH JUNCKER AGENT FRACTIONATION OF AIRTO OBTAIN OXYGEN OF ABOUT SEVENTY PERCENT PURITY BACKGROUND OF THEINVENTION This invention relates to a process of obtaining oxygen ofapproximately 70 percent purity by two-stage rectification of air in amedium-pressure column and a low-pressure column and by work-performingexpansion of a process stream to the pressure of the lowpressure column,in which process the incoming air is cooled against fractionationproducts and is divided into at least two streams, one of which isconducted directly into the lower part of the medium-pressure column andanother into the low-pressure column.

It is known that the separation of gas mixtures into their individualcomponents requires a greater expenditure of energy the greater thedesired purity of the products is. It is furthermore known that,referred to the nitrogen separated from the air, a minimum consumptionof energy results when oxygen of a purity of about 60 to 80 percent isproduced (Handbuch der Kaltetechnik, Vol. 8, published bySpringer-Verlag, 1957, pages 196-197).

Since the metallurgical industry in particular uses large quantities ofoxygen-enriched air, it has been endeavored for a long time to developmethods in which the minimum energy consumption is economicallyutilized. A prerequisite for this is that themethods operate with thelowest possible initial pressure of the air which is to be fractionated.In distillative separation it is difficult in this connection to makeavailable sufficient liquid nitrogen as wash liquid at the head of therectification column, since the temperature of condensation of thenitrogen decreases with a decrease in the pressure.

Refrigeration for the condensation of the nitrogen is available inparticular as a result of the evaporating oxygen product. Since thepressure of the evaporating oxygen product cannot be lowered withoutlimit unless it is withdrawn from the plant by a vacuum pump, theevaporation temperature of the oxygen product is also fixed.

Up to now in principle two methods have been known which make itpossible to-get along with the lowest possible initial pressure of theair. These are, on the one hand, the recycle methods, under which alsothe 'so-called two-pressure methods must be included, As examplesmention may be made here of German Pat. No. 1,187,248 and British Pat.No. 1,033,931.

In these recycle methods a part of the wash nitrogen is more easilyliquefied under a somewhat higher pressure. Such methods aredisadvantageous as they are relatively expensive from the standpoint ofthe apparatus. They require a second compressor and additional recycleheat exchangers, whereby additional loss of flow and cold result. In thetwopressure methods, a more complicated air compressor as well asexpensive heat exchangers are required.

In the other method, so-called parallel-flow evaporators or columns areused for fractional condensation. As examples mention may be madeofGerman Pat. No. 1,177,658 and German Application No. 1,934,755. Inthese methods, the impure oxygen does not evaporate at constanttemperature but rather over a temperature range. In a normalrectification column, the cooling power is made available at the head ofthe column at the temperature prevailing there. This industrially is thesimplest solution but it is not optimum from a thermodynamic viewpointsince more ability to perform work (energy) is expended than is actuallynecessary. These methods attempt to make the cooling power availableoverv the sliding temperature range of the rectification column. Forthese methods, special rectification columns are necessary for whichthere has not been available a fully satisfactory industrial solution.Since, for reasons of safety, the oxygen product must not experience dryevaporation, the effect of the sliding evaporation temperature cannot befully utilized. Hence, for reasons of safety, the design possibilitiesand the operation of such special columns are limited.

Theobject of the present invention is to provide a method which is atleast as favorable with respect to the consumption of energy as theknown methods but operates with well-proven, ordinary apparatuscomponents and without expenditure for special apparatus. In particular,the advantages of sliding evaporation of the oxygen product without useof a special column are to be utilized, and the method is to requirecompression of the inlet air to a single pressure level and norecycling.

SUMMARY OF THE INVENTION .According to this invention, oxygen ofapproximately percent purity is obtained by a two-stage rectificationprocess in which the incoming air is cooled against the fractionationproducts and divided into at least two streams, one of which isconducted directly into the lower part of the first or medium-pressurecolumn and the other stream is condensed againstevaporating liquidimpure oxygen withdrawn from the second or lowpressure column. Thecondensed air stream is further cooled to a low temperature, expanded,evaporated against the vapor of the top of the medium-pressure columnand thereupon conducted into the low-pressure column. Furthermore, thetop of the medium-pressure column is cooled by the crude oxygen from thebottom of that column and a process gas stream is expanded with theperformance of work to the pressure of the low-pressure column.

It is advantageous if a part of the stream ofair, which has beencondensed by the evaporation of liquid impure oxygen from thelow-pressure column, is conducted as additional liquid feed into themediumpressure column. Similarly, a part of the condensed air stream,after being subcooled, can be branched off and expanded directly intothe low-pressure column. In this way energy is saved, since with thismethod the rectification operates with only slight disturbances inequilibrium.

As the process stream which is to be expanded with the performance ofwork, there is advantageously employed a part of the incoming air or agas fraction obtained from the first column.

BRIEF DESCRIPTION OF THE DRAWINGS The invention willnow be furtherdescribed with ref- I pressure column is expanded with the performanceof work; and

FIG. 3 is a flow sheet of still another embodiment in Y which a gaseousintermediate fraction is withdrawn DESCRIPTION OF THE PREFERREDEMBODIMENTS:

In the process shown in FIG. 1, air compressed to about 3.5 ata(atmospheres absolute) passes at room temperature through line 5 intoheat exchangers 6 and 7 and into gas-phase filter 8 in whichhydrocarbons contained in the air and traces of carbon dioxide whichwere not frozen-out in heat exchangers 6 and 7 are retained. The air iscooledin heat exchangers 6 and 7 to close to the dew point.

After gas-phase filter 8 the air is divided into two streams. About 20percent of the incoming air flows through line 10 into heat exchanger 7and is warmed somewhat therein. If desired, this air can in part alsobypass heat exchanger 7 via valve 1 lb. Via bypass line 11a, a part ofthe reheated air can be fed through regulating heat exchanger 12 so thatthe air finally enters turbine 13 with a temperature of about l68C andis there expanded to 1.32 ata, the pressure of lowpressure column 2. Theair cooled by the workperforming expansion then, flows through line 14into low-pressure column 2.

The other stream, constituting about 80 percent of the incoming air,flowsthrough air precondenser 16 and is then divided into second andthird streams. The second stream, constituting about 50 percent of thetotal air, passes through line 9 directly into mediumpressure column 1.This air is fed above the bottom liquid pool in column 1 and providesvapor upflow therein.

The third stream, about 30 percent of the total air, passes through lineinto air condenser 17 in which it condenses. A small part of thecondensed air passes via line 18 into medium-pressure column 1 and therefortifies the reflux. The main quantity of the condensed air whichcontinues through line I5 is cooled to about -1 89C in subcooler 4 andsplit into streams of approximately equal size in lines 19 and 20.

The stream flowing through line 19 is expanded in throttle valve 21ainto the upper part of low-pressure I column 2. The stream in line 20 isexpanded in throttle valve 21b to the pressure of low-pressure column 2and y is evaporated in condenser 3,. whereby it assists in cooling vaporfrom the top of medium-pressure column 1. The evaporated air combineswith the expanded air from turbine 13 in line 14 and is introducedtogether with the latter into the lower third of low-pressure column 2.

Medium-pressure column 1 fractionates the air fed thereto into nitrogenand crude oxygen which is obtained in the bottom of column 1 in liquidform containing about 41 percent oxygen. The nitrogen is withdrawn ingaseous form by line 22 from the top of medium-pressure column 1 and iscondensed in condenser 3. A part of the condensed nitrogen passesthrough line 23 as reflux back into medium-pressure column 1. Thebalance of the condensed nitrogen passes through line 24 into nitrogensubcooler 25 and then is introduced via throttle valve 26 as reflux intolow-pressure column Liquid crude oxygen is withdrawn from the bottom ofmedium-pressure column 1 through line 27 and subcooled in cooler 4; itis then expanded in throttle valve 28 and evaporated in condenser 3,whereby nitrogen vapor from the top of medium-pressure column 1 iscondensed. In gaseous form,'the crude oxygen then flows through line'27into low-pressure column 2 above the liquid pool in the bottom andproduces a gaseous upflow' in column 2.

Low-pressure column 2 effects the final fractionation. From its top,through line 29 there escapes gaseous nitrogen which after flowingthrough subcoolers 25 and 4, air precondenser l6, regulating heatexchanger 12 and heat exchangers 7 and 6, leaves the plant at roomtemperature. Subcooler 25 is provided with bypass line 30 and regulatingvalve 31 for the gaseous nitrogen flowing from low-pressure column 2through line 29.

From the bottom of low-pressure column 2, liquid oxygen of about percentpurity is withdrawn through line 32 and expanded to nearly atmosphericpressure in throttle valve 33. Thereupon, it passes into condenser 3where it partially evaporates, thus taking up about one-third of theheat necessary for its evaporation. This heat is withdrawn from thevapor from the top of medium-pressure column 1. Separation of thisoxygen stream into phases then takes place in separator 34. The liquidphase is withdrawn through line 35 and is pumped by circulating pump 36through oxygen filter 37 where any-hydrocarbons still present in theliquid are retained by adsorption.

About two-thirds of the circulated liquid phase then evaporates in aircondenser 17, while air stream 15 flowing in the opposite direction iscompletely condensed. The remainder of the liquid oxygen which has beenabout two-thirds evaporated passes back into separator 34. Gaseousoxygen of about 70 percent purity leaves separator 34 through line 39.This gaseous oxygen gradually gives off its remaining cold in airprecondenser 16, regulating heat exchanger 12 and heat exchangers 7 and6. Thus, the plant yields as product, gaseous oxygen of about 70 percentpurity, at room temperature and atmospheric pressure.

By the method of this invention, it is possible to use a large amount ofthe refrigeration in the impure liquid oxygen from low-pressure column 2to cool vapor from the top of medium-pressure column 1.

The impure liquid oxygen can, to be sure, be evaporated only to theextent of about one-third in condenser 3 since with evaporation itbecomes warmer and warmer due to the enrichment of oxygen in theremaining liquid, but the unevaporated portion can' be evapo ratedagainst the incoming air in line 15. The air in line 15 is thuscondensed and refrigeration originally in the impure liquid oxygen cannow be given off near the top of medium-pressure column 1 byre-evaporation of condensed air introduced through line 18. The impureliquid oxygen could be evaporated completely in condenser 3 only if thepressure of medium-pressure column 1 were increased. Theturbo-compressor (not shown) for the air flowing into the plant wouldthen require more energy.

In the process of FIG. 2, the incoming air is divided into only twostreams. The stream which in the process of FIG. 1 was expanded with theperformance of work in turbine 13 is eliminated. In its place, a partialstream of the nitrogen withdrawn in gaseous form by line 22 from the topof medium-pressure column 1 is passed into line 40, heated in airprecondenser l6 andheat exchangers 12 and 7, and expanded with theperformance of work in turbine 41. The expanded, cooled nitrogen flowsthrough line 42 and is combined with the nitrogen withdrawn through line29 from the top of low-pressure column 2. As in the process of FIG. 1,bypassline l la and valve 1112 are also provided up-stream of turbine41.

The process of FlG.'3 is substantially similar to that of FIG. 2.However, gaseous nitrogen from the top of medium-pressure column 1 isnot expanded, but rather an intermediate gaseous fraction with aboutpercent oxygen is removed by line 45 from medium-pressure column 1 atthe level where liquid air is charged through line 18. The intermediatefraction then flows through air precondenser l6 and heat exchangers l2and7. After expansion with performance of work in turbine 43, theintermediate fraction passes through line 44 into low-pressure column 2.It enters there at the level where liquid air is introduced through line19.

What is claimed is:

1. The process for obtaining oxygen of approximately 70 percent purityby the two-stage rectification of air and by the expansion of a processgas stream with the performance of work, which comprises cooling theincoming air by indirect heat exchange with the products ofrectification, dividing the cooled air into at least two unfractionatedstreams, discharging one of said unfractionated streams into the lowerpart of the first medium-pressure column, condensing another of saidunfractionated streams by indirect heat exchange with evaporating liquidoxygen of approximately 70 percent purity withdrawn from the secondlow-pressure column, sub-cooling and then expanding thus condensedunfractionated air which is then evaporated by indirect heat exchangewith the top vapor of said first column, discharging thus evaporatedunfractionated air into the lower part of said second column, andevaporating the bottom liquid of said first column by indirect heat ex-4. The process of claim 1 wherein the liquid oxygen of approximatelypercent purity withdrawn from the second column is partially evaporatedby indirect heat exchange with top vapor of the first column prior tocompletion of evaporation by indirect heat exchange with the air streamcondensed thereby.

5. The process of claim 1 wherein the stream discharged into the lowerpart of the first column is at least 50 percent of the cooled air at apressure of about 3.5 ata and part of the condensed air stream, afterbeing subcooled, is expanded and discharged directly into an'upper levelof the second column as additional reflux liquid.

6. The process of claim 1 wherein the condensation of the air stream byindirect heat exchange with evaporating liquid oxygen of approximately70 percent purity is carried out by pumping said liquid oxygen throughsaid heat exchange to effect only partial evaporation thereof, theresulting vapor and liquid are separated, and the separated liquid iscombined with said liquid oxygen being pumped through said heatexchange.

7. The process of claim 6 wherein the liquid oxygen of approximately 70percent purity withdrawn from the second column is partially evaporatedby indirect heat exchange with top vapor of the first column prior tocompletion of evaporation by indirect heat exchange with the air streamcondensed thereby.

8. The process of claim 7 wherein the stream discharged into the lowerpart of the first column is at least 50 percent of the cooled air at apressure of about 3.5 ata and part of the condensed air stream, afterbeing subcooled, is expanded and discharged directly into an upper levelof the second column as additional reflux liquid.

2. The process of claim 1 wherein part of the condensed air stream isdischarged into an upper level of the first column as additional refluxliquid.
 3. The process of claim 1 wherein part of the condensed airstream, after being subcooled, is expanded and discharged directly intoan upper level of the second column as additional reflux liquid.
 4. Theprocess of claim 1 wherein the liquid oxygen of approximately 70 percentpurity withdrawn from the second column is partially evaporated byindirect heat exchange with top vapor of the first column prior tocompletion of evaporation by indirect heat exchange with the air streamcondensed thereby.
 5. The process of claim 1 wherein the streamdischarged into the lower part of the first column is at least 50percent of the cooled air at a pressure of about 3.5 ata and part of thecondensed air stream, after being subcooled, is expanded and dischargeddirectly into an upper level of the second column as additional refluxliquid.
 6. The process of claim 1 wherein the condensation of the airstream by indirect heat exchange with evaporating liquid oxygen ofapproximately 70 percent purity is carried out by pumping said liquidoxygen through said heat exchange to effect only partial evaporationthereof, the resulting vapor and liquid are separated, and the separatedliquid is combined with said liquid oxygen being pumped through saidheat exchange.
 7. The process of claim 6 wherein the liquid oxygen ofapproximately 70 percent purity withdrawn from the second column ispartially evaporated by indirect heat exchange with top vapor of thefirst column prior to completion of evaporation by indirect heatexchange with the air stream condensed thereby.
 8. The process of claim7 wherein the stream discharged into the lower part of the first columnis at least 50 percent of the cooled air at a pressure of about 3.5 ataand part of the condensed air stream, after being subcooled, is expandedand discharged directly into an upper level of the second column asadditional reflux liquid.